I use an EF 300 f2.8L II ii, with and without extenders 1.4x and 2x, both v.iii, on a 5D iii.All combinations have been micro-adjusted using Reikan Focal software.

I generally have pretty sharp images. But sometimes, I feel the bokeh from the wide open 300 + TC 2x very "dirty", both in front of and behind the depth of field area, despite the target remains sharp enough. This is not always happening, but more systematically when the target is quite far (above 50 meters).

I do not notice the same behavior with the wide open 300 + TC 1.4x.

I indeed have 2 questions :

1) Is there a recommended target distance for using the wide open 300 + TC 2x, beyond which the image quality would degrade fast ?

2) Would you maybe advice to replicate AF micro-adjustement at a longer distance (50 m for example) than the 10x to 50x the focal length which is usually recommended (I did it at about 18 m, because my garden cannot allow me more) ?

Bohns, the bokeh with extenders isn't going to be as pretty as a raw lens, but it depends a lot on the subject in the out of focus area and how far it is from your subject. Generally complex textures like small leaves and thick grasses can look odd if they are right next to your subject.

I find that f/8 is noticeably sharper with this combination, but also used Focal to perform AFMA. I don't have a ton of room in my house, so I did the calibration at roughly 30ft./10m, and obtained good results. I'm pretty sure I've read in Focal or Canon's literature that the 50x distance is more important for shorter focal lengths and as long as you were reasonably past MFD you can obtain good AFMA. I have found that my results often better at around 20-30x, versus 50x due to the target size, as you say.

I used a solid tripod/head weighed down with 20lbs of sandbags, wireless remote, very well-lit target (+11EV) and did a Manual calibration at -20,-15--10,-5,0,5,10,15,20 and after running the test, re-ran it at +5 through +9 to get an even more precise calibration (of +8) from the +7 it recommended during the first test.

I generally try to close a bit aperture down to f6.3, f7.1 or f8, and it is true image quality and bokeh improve then. But sometimes, lighting conditions require wide aperture at f5.6.

Nobody to advise a "maximum" recommended shooting distance beyong which the image quality of the wide open EF300 f2.8 IS ii + TC 2x iii combo would degrade fast ?

Some people in another forum said max 50 meters. But I feel this short...

I understand and don't hesitate to use f/5.6 if needed - you can always sharpen a bit more in post. As for a recommended maximum distance, I'm not aware of any limitations of the lens or lens+extender combo. What you may experience is environmental issues softening the image - haze, humidity, heat, etc. These can all degrade images with (super) telephoto lenses. Where I live, it is very hot & humid most of the year, and all of the shots I've tried at 100m+ are all blurred & wavy due to the "mirage effect" of the heat. In cold, clear conditions, you should be able to shoot at fairly extended distances without any issues, though.

I understand and don't hesitate to use f/5.6 if needed - you can always sharpen a bit more in post. As for a recommended maximum distance, I'm not aware of any limitations of the lens or lens+extender combo. What you may experience is environmental issues softening the image - haze, humidity, heat, etc. These can all degrade images with (super) telephoto lenses. Where I live, it is very hot & humid most of the year, and all of the shots I've tried at 100m+ are all blurred & wavy due to the "mirage effect" of the heat. In cold, clear conditions, you should be able to shoot at fairly extended distances without any issues, though.

Thanks again Mackguyver !Atmospheric conditions are factually good explanations.

Prior to this answer, I anyway questioned Canon France. I wait for their answer and will keep the forum informed.